1. Field of the Invention
The present invention relates to an article of manufacture for conducting electrical signals. In particular, a guarded coaxial cable is provided for conducting radio frequency signals.
2. Discussion of the Related Art
Coaxial cables used for television including satellite, cable TV and antenna cables are typically 7 mm in diameter, a size large enough to limit signal loss over the distances traveled from an outside location to a location inside a home or building. Usually these cables originate outside a home or apartment such as a multiple dwelling unit (MDU) and terminate inside where TV, wireless, or satellite reception equipment is located.
A cable often enters a building through a hole drilled in a wall. But, drilling a hole in a wall and routing a cable through the hole makes a permanent alteration to the building. Since most MDU occupants do now own the premises, this simple action raises issues including unauthorized building modifications, ownership of cable modifications, liability for changes, and liability for related safety issues.
Wireless solutions do not solve this problem. While capacitive coupling solves the problem of transporting high frequency signals across a glass boundary, such wireless solutions are unable to transport mid and low frequency signals. In particular, cable and satellite television signals, electric powering of outdoor devices, and low frequency control signals must be transported using electrical conductors such as coaxial cables.
A solution using the space between a window or door and an adjacent frame is well known. Here, cables are passed through an existing opening without modification to the building structure. But, using such openings to pass a typical 7 mm O.D. coaxial cable presents challenges including; closing the window or door when it is blocked by the cable; and, maintaining a fully functional cable when it is deformed by impact and compression from operation of the window or door.
The gap between a window/door and its frame is typically less than the 7 mm size of the cable. In many windows and doors, the space provided for soft weather sealing material and/or the latching tolerance of the door/frame interface provides a gap on the order of about 3 mm. Therefore, a 7 mm coaxial cable in this application will likely be squeezed and damaged while a cable of 3 mm or smaller diameter will likely avoid damage.
Coaxial cable deformations are undesirable because they damage cable covering and abruptly change the coaxial cable conductor spacing. In particular, conductor spacing changes tend to alter the characteristic impedance of the cable and reflect radio frequency power back toward the source, causing a condition called standing waves. An abrupt change in impedance acts as a signal bottleneck and may result in detrimental data delays and signal lock-ups found in satellite TV signal transmission systems.
Coaxial cable entry solutions face a variety of problems including one or more of: 1) traveling through a small space between the closed window/door and its frame; 2) destruction or degradation from impacts when windows or doors are operated; 3) functioning within its specifications, for example a DBS Satellite coaxial cable must maintain a minimum impedance matching of the RF signal (12 dB minimum return loss at 2150 MHz) in order for the home device to operate correctly; and, 4) passing electric current such as a DC current to power an outside device and low frequency control signals when needed.
The present methods of solving these problems lie in the construction of an extension cable that can pass through the small space and have coaxial connectors at each end to re-fasten the larger 7 mm coaxial long distance transmission cable at each end. These methods include using coaxial cables with diameters in the range of 3-4 mm, using armor such as metallic armor and other armoring methods known to persons of ordinary skill in the art, and flattening a coaxial cable to provide a thin profile.
None of these methods provides a robust solution. The first method often fails to protect the cable since cables over 3 mm in diameter are larger than typically available window/door to frame gaps. When the door or window is closed, these cables are deformed to varying degrees rendering them useless and/or degrading their RF performance. In addition, the outer covering on such cables is soft and easily breached by repeated operation of windows/doors.
The second method using armor not only uses cables larger than 3 mm, it also prevents the cable from making sharp turns such as 90 degree bends typical of the window and door frame applications. Here, the minimum bending radius of the extender cable is unacceptably increased by the armor.
The third method using a flattened/non-circular coaxial cable provides inferior RF performance even before it is installed. In addition, bending the flat coaxial cable to accommodate one or more sharp bends of window/door frames further distorts the cable cross-section and impairs signal transmission. Further, the soft sheath of a coaxial cable can easily be breached by repetitive impacts from operation of windows/doors.
What is needed is a guarded coaxial cable assembly having features including one or more of the following: 1) a cable assembly providing good RF performance including meeting industry standards such as 10 dB return loss, for a 75 ohm impedance, at a highest frequency of about 2150 MHz; 2) the cable assembly safely passing DC currents up to about 1.5 amperes with acceptable and/or minimal loss; 3) the cable assembly able to make multiple 90 degree bends to fit into the door frame; and, 4) the cable assembly performing within its specifications despite repeated impacts from windows/doors.
While known solutions are widely employed and the cable and satellite television industry shows little interest in developing new solutions, the present invention offers significant advancements over what has been done before.